Mysql核心点
对于每一位研发同学，Mysql都是必须掌握的技能啦。基本的Mysql的操作，还是得很好的掌握的。
一、Mysql
学习一个技术，一定要先去官网学习。
Mysql官网
二、基本的查询
1.创建表并插入数据
创建表
CREATE TABLE shop (
article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
dealer CHAR(20) DEFAULT '' NOT NULL,
price DOUBLE(16,2) DEFAULT '0.00' NOT NULL,
PRIMARY KEY(article, dealer));
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插入数据
INSERT INTO shop VALUES
(1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),
(3,'C',1.69),(3,'D',1.25),(4,'D',19.95);
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查看这张表
SELECT * FROM shop;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0001 | A | 3.45 |
| 0001 | B | 3.99 |
| 0002 | A | 10.99 |
| 0003 | B | 1.45 |
| 0003 | C | 1.69 |
| 0003 | D | 1.25 |
| 0004 | D | 19.95 |
+---------+--------+-------+
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2.查询一列中的最大值
SELECT MAX(article) AS article FROM shop;
+---------+
| article |
+---------+
| 4 |
+---------+
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3.查询这一列中最大值所在的行
通过子查询的方式，能够轻松操作。
SELECT article, dealer, price
FROM shop
WHERE price=(SELECT MAX(price) FROM shop);
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0004 | D | 19.95 |
+---------+--------+-------+
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其他的解决方案，例如使用LEFT JOIN，或者根据指定的元素进行排序，并且使用特定的Mysql的LIMIT语句进行操作。
方案1：
SELECT s1.article, s1.dealer, s1.price
FROM shop s1
LEFT JOIN shop s2 ON s1.price < s2.price
WHERE s2.article IS NULL;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0004 | D | 19.95 |
+---------+--------+-------+
方案2：
SELECT article, dealer, price FROM shop ORDER BY price DESC LIMIT 1;
这里使用了Limit值为1，意为只取一个值。
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4.找到每篇文章的最大值，如果一篇文章有多个值，只取最大值
SELECT article, MAX(price) AS price
FROM shop
GROUP BY article;
+---------+-------+
| article | price |
+---------+-------+
| 0001 | 3.99 |
| 0002 | 10.99 |
| 0003 | 1.69 |
| 0004 | 19.95 |
+---------+-------+
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5.求出每篇文章，价格最高对应的经销商
方法1：用自查询
SELECT article, dealer, price
FROM shop s1
WHERE price=(SELECT MAX(s2.price)
FROM shop s2
WHERE s1.article = s2.article);
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0001 | B | 3.99 |
| 0002 | A | 10.99 |
| 0003 | C | 1.69 |
| 0004 | D | 19.95 |
+---------+--------+-------+
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但是使用自查询是效率比较低下的一种方式。
那么另外的解决方案，是在FROM中使用子查询或者LEFT JOIN。
SELECT s1.article, dealer, s1.price
FROM shop s1
JOIN (
SELECT article, MAX(price) AS price
FROM shop
GROUP BY article) AS s2
ON s1.article = s2.article AND s1.price = s2.price;
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LEFT JOIN：
SELECT s1.article, s1.dealer, s1.price
FROM shop s1
LEFT JOIN shop s2 ON s1.article = s2.article AND s1.price < s2.price
WHERE s2.article IS NULL;
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6.使用用户自定义的变量查询
您可以使用MySQL用户变量来记住结果，而无需将它们存储在客户端的临时变量中。
例如，要查找价格最高和最低的文章，您可以执行以下操作：
mysql> SELECT @min_price:=MIN(price),@max_price:=MAX(price) FROM shop;
mysql> SELECT * FROM shop WHERE price=@min_price OR price=@max_price;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0003 | D | 1.25 |
| 0004 | D | 19.95 |
+---------+--------+-------+
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